The materials surrounding the occupants of a building are of prime importance for protection against heat and cold. Great care must be taken in the choice of the wall and roof materials and their thicknesses with respect to their physical properties, such as thermal conductivity, resistivity and transmission, and optical reflectivity.
Considering an external wall exposed to a high outside air temperature and a lower inside air temperature , the rate of heat flow transmitted through the wall from the outside air to the inside air is proportional to the air temperature difference, area of the wall, and rate of global heat transmittance that can be determined from an analysis of the components of the total resistance to heat flow. The total resistance is composed of the resistance to heat flow through the material, the interfacial resistance at the external surface, and the interfacial resistance at the internal surfaces. Since the interfacial resistances are determined primarily by temperature conditions over which the builder has little control, his principal effect on the heat transmittance is on changing the resistance to heat flow through the wall material. To reduce the heat transmission from one side of a wall to the other, the thermal transmittance must be reduced as much as possible by either increasing the thickness of the wall or using materials of lower thermal conductivity and therefore of higher resistance. Often walls composed of several materials, as shown in figure 2, are used to provide the desired thermal and aesthetic wall characteristics. Coefficients of thermal transmittance for a variety of wall materials and of combinations of such materials are provided in Appendix 3. These coefficients are given in the practical units commonly used: kcal/hm²C° and Btu/hft²F°.
In hot arid climates, the coefficient of thermal transmittance should be about 1.1 kcal/hm²C° (0.225 Btu/hft²F°) for an outer wall to have an appropriate thermal resistance. Table 1 lists the thicknesses of walls composed of various construction materials needed to achieve coefficients of approximately 1.1 kcal/hm²C° (0.225 Btu/hft²F°).
These tables do not contain data for mud-brick walls. However, experiment has proved that mud brick is most appropriate for achieving thermal comfort in addition to being widely available to all segments of the population.
In 1964, six small experimental buildings were built on the grounds of the Cairo Building Research Centre, using different materials. They were used to evaluate cost, local availability, and thermal comfort. Two modes of these six represented extremes. One was built entirely of mud brick with the 50-cm (20-inch) thick walls and roof in the shape of a combined dome and vault. The other was built of 10-cm (4-inch) thick prefabricated concrete panels for both the walls and the roof. Plans and sections of these buildings are given in figures 5 and 6, respectively.
These models were examined on a day in March when external air temperature varied from 12 °C (53.6 °F) at 6 A.M. to 28 °C (82.4 °F) at 2 P.M. and back to 12 °C (53.6 °F) at 4 A.M. As shown in figure 7, the airtemperature fluctuation inside the mud-brick model did not exceed 2 C° (3.6 F°) during the 24-hour period, varying from 21-23 °C (69.8-73.4 °F), which is within the comfort zone. However, the maximum air temperature inside the prefabricated model reached 36 °C (97 °F), or 13 C° (23 F°) higher than in the mud-brick model and 9 C° (16 F°) higher than the outdoor air temperature. It fell within the comfort zone for only one hour in the morning (9-10 A.M.) and between 8:40 P.M. and 12:20 A.M., as recorded in figure 8. The contrast can be explained by the fact that concrete has a thermal conductivity of 0.9, while that of mud brick is 0.34, and that the mud-brick wall is five times thicker than the prefabricated panels. Thus, the mud-brick wall has a thermal resistance more than 13 times greater than the prefabricated concrete wall. Unfortunately, these models were not evaluated for the salient dates of the equinoxes and solstices, which would have provided complete information, especially about the lag effect and heat storage.
Table 1. Thicknesses of walls of different material that give coefficients of thermal transmittance of approximately 1.1 kcal/hm²C° (0.225 Btu/htt²F°)
|(in m)||(in in)||(in kcal/ hm²C°)||(m Btu/ hft²F°)|
|Hollow brick block||0.30||12||1.10||0.225|
|Double-wall brick with holes and 8-cm cavity||2 x 0.12||2 x 4.7||1.12||0.229|
|Brick wall with holes||0.38||15||1.03||0.211|
|Hollow block sand-lime brick||0.51||20||1.16||0.238|